The invention relates generally to obtaining measurements for optical characteristics of a device under test and more particularly to reducing the effects of vibration noise on the process of obtaining the measurements.
Techniques for testing or analyzing optical components are currently available. A xe2x80x9cdevicexe2x80x9d under test (DUT), such as a length of fiberoptic cable, may be tested for faults or may be analyzed to determine whether the device is suitable for use in a particular application. System components such as multiplexers, demultiplexers, cross connectors, and devices having fiber Bragg gratings may be separately tested before they are used in assembling a system.
Optical testing may be performed using a heterodyne optical network analyzer. Such analyzers are used for measuring optical characteristics of optical components. For example, the xe2x80x9cgroup delayxe2x80x9d of a component may be important in determining the suitability of the component for a particular system. Group delay is sometimes referred to as envelope delay, since it refers to the frequency-dependent delay of an envelope of frequencies. The group delay for a particular frequency is the negative of the slope of the phase curve at that frequency.
Typically, a heterodyne optical network analyzer includes two interferometers. FIG. 1 is an example of one type of heterodyne optical network analyzer 10. The analyzer includes two interferometers 12 and 14 connected to a tunable laser source (TLS) 16. The TLS generates a laser light beam that is split by a coupler 18. The TLS is continuously tuned, or swept, within a particular frequency range. By operation of the coupler 18, a first portion of the coherent light from the TLS is directed to the DUT interferometer 12, while a second portion is directed to the reference interferometer 14.
The DUT interferometer 12 has a second coupler 22 that allows beam splitting between a first arm 24 and a second arm 26. A mirror 28 is located at the end of the first arm and a DUT 20 is located near the reflective end of the second arm. The lengths of the two arms can differ, and the difference in the optical path length is represented in FIG. 1 by LDUT. Since the DUT can be dispersive, the actual optical path length is a function of frequency. A detector 30 is positioned to measure the combination of the light reflected by the mirror 28 and the light reflected by the DUT 20. Processing capability (not shown) is connected to the detector. Assuming the two arms 24 and 26 have different lengths, the light from one of the arms will be delayed by a time T1 with respect to light from the other arm. Generally, T1 varies as a function of frequency, since the DUT is typically dispersive. The two beam portions interfere when they recombine at the coupler 22. By analyzing the signal that is generated at the detector 30, the group delay and other properties of the DUT may be determined. However, in order to very precisely measure the group delay, it is necessary to obtain knowledge of the frequency tuning of the TLS 16 as a function of time. The reference interferometer 14 is used for this purpose.
The structure of the reference interferometer 14 is similar to that of the DUT interferometer 12, but a mirror 32 takes the place of the DUT 20. A second detector 34 receives light energy that is reflected by the combination of the mirror 32 at the end of a third arm 36 and a mirror 38 at the end of a fourth arm 40. As in the DUT interferometer, the lengths of these two arms 36 and 40 can be different, and this difference in lengths is represented by LREF. The signal that is generated by the second detector 34 is also an interference signal (i.e., an intensity signal having an interference term) that is responsive to the combination of light from the two arms. However, the optical characteristics of the reference interferometer are relatively fixed and therefore predictable. Consequently, the reference interferometer can be used to measure the major variable to its operation, i.e., frequency sweep xcfx89(t).
A concern is that vibrations to the system will diminish the precision of measurements such as group delay, group velocity, transmissivity, reflectivity, and chromatic dispersion. For example, vibrations of the second arm 26 on which the DUT 20 resides will act to change the index of refraction of the arm, which in turn acts as perturbations to the phase delay measured by processing the signal from the detector 30. The effects of vibrations on the precision of such measurements similarly occur in transmission-type interferometers, where an interference signal is formed as the combination of two beam portions that have propagated through the two arms of an interferometer without reflection. Thus, detectors are at the ends of the arms opposite to the TLS that generates the original beam. Transmission-type interferometers, such as Mach-Zehnder interferometers, are well known in the art.
One method of addressing the vibration concern is to provide vibration isolation of the heterodyne optical network analyzer 10. For example, the system may be supported on a platform that is specifically designed to minimize vibrations. However, additional or substitute techniques are desired. What is needed is a method and system for significantly reducing the risk that vibrations will adversely affect the performance of an interferometer.
In accordance with the invention, vibration noise within an interferometric system has reduced effects as a result of monitoring light patterns and providing corrections on the basis of the light patterns. Light propagating through a first path of the system is combined with light propagating through a second path to form at least one interference signal. Within each of the embodiments of the invention, the combination of light from the two paths is analyzed to provide a basis for the corrections.
In one embodiment, a partial reflector is added to an interferometer for analyzing a device under test (DUT), such as a fiber optic cable or the like. A source of a sweeping frequency beam is coupled to the two paths, or arms, so that beam portions are introduced to the two paths. As one possibility, the source of the beam is a tunable laser source (TLS). The DUT and the partial reflector are connected in close proximity along one of the paths. Therefore, vibrations experienced by the DUT are likely to be experienced in generally equal magnitude by the partial reflector, so that the vibration noise effects of the two components will be generally equal. Moreover, the radian frequencies at the two components will remain substantially the same as the TLS sweeps through its frequency range. With these approximations, the effects of vibration can be reduced by using techniques such as determining the phase difference between the phase of the interference signal for the DUT and the phase of the interference signal for the partial reflector. This phase difference can then be applied in known approaches to determining optical characteristics of the DUT, such as measurements of group delay, group velocity, transmissivity, reflectivity and chromatic dispersion.
The use of the partial reflector works well in reflection interferometers, i.e., interferometers in which the interference signal is formed of reflected light from the two paths. However, the same approach may be used in a transmission interferometer in which the interference signal is formed by combining light that has propagated through the two paths. The first path having the DUT may include a shunt in parallel with the DUT. Reduction of vibration noise during analysis of the DUT can be achieved if the shunt is located so that it is likely to experience the same vibrations as the DUT. When the shunt and the DUT are approximately the same length (but not exactly the same length) and experience approximately the same magnitude of vibration, the effects of the vibrations on measurements of the optical characteristics of the DUT can be reduced. The shunt and DUT should not be exactly the same length, since such an arrangement would cause interference specific to propagation through the shunt to be indistinguishable from interference specific to propagation through the DUT.
As another alternative to using partial reflectors, measurements of Rayleigh Backscatter can be considered. Since the section of optical fiber closest to the DUT would experience approximately the same vibration noise as the DUT, the ideal selection of the Rayleigh Backscattered signal to be considered is the signal section that corresponds to the path region closest to the DUT. Known techniques may be used to filter a portion of the interference signal data to calculate the vibration noise of the section. The calculated vibration noise can be subtracted from the DUT response in much the same manner as the reflections from the partial reflector are used.
In yet another embodiment, the effects of vibration are reduced by providing the TLS as the means for measuring the optical characteristics of the DUT, but adding a second source of light as a means for measuring vibrations. The second source may be a fixed frequency source, with the frequency being outside of the frequency range of the TLS. However, other arrangements are contemplated. Both the TLS and the fixed frequency source of light provide beam portions that are propagated along the two paths, but then recombined to form the interference signals. Particularly, where the fixed frequency is outside of the frequency range of the TLS, it is easily possible to distinguish the interference signal of the fixed frequency light from the interference signal of the sweeping frequency light. Typically, wavelength filtering is used to distinguish the two interference signals. Since the frequency of the added interference signal is fixed, changes are most likely to be a result of vibrations. Therefore, monitoring the changes in the interference signal allows the vibration noise to be isolated in the interference signal of the TLS light. In one application, the phase of the fixed frequency interference signal is tracked and is used in providing an offset in the calculation of the optical characteristics of the DUT, such as in calculating group delay. In another application, calculations with respect to the fixed frequency interference signal are used to provide mechanical adjustments, rather than calculation adjustments. As an example, the length of one of the two paths may be dynamically adjusted to offset the effects of vibrations. A piezoelectric mechanism may be dynamically controlled to change the length of the second arm to compensate for the vibration-induced xe2x80x9cchangesxe2x80x9d in length of the DUT arm (since vibrations cause xe2x80x9ceffectivexe2x80x9d length changes).
An advantage of the invention is that much of the error that occurs as a result of vibrations of an interferometric system is eliminated using the described approaches. These approaches may be used in combination with known techniques, such as mounting the system on a vibration-isolation platform. More accurate and reliable determinations of the optical characteristics of a DUT can then be achieved.